A single laser beam brought to a focus with a strongly converging lens forms a type of optical trap widely known as an optical tweezer. Multiple beams of light passing simultaneously through the lens' input pupil yield multiple optical tweezers, each at a location determined by its beam's angle of incidence and degree of collimation at the input pupil. The trap-forming laser beams form an interference pattern as they pass through the input pupil, whose amplitude and phase corrugations characterize the downstream trapping pattern. Imposing the same modulations on a single incident beam at the input pupil would yield the same pattern of traps, but without the need to create and direct a number of independent input beams. Such wavefront modification can be performed by a type of diffractive optical element (DOE) commonly known as a hologram.
Holographic optical trapping (HOT) uses methods of computer-generated holography (CGH) to project arbitrary configurations of optical traps, with manifold applications in the physical and biological sciences, as well as in industry. This flexible approach to manipulate and transform mesoscopic matter has been used to assemble two-and three-dimensional structures, to sort objects ranging in size from nanoclusters to living cells, and to create all-optical microfluidic pumps and mixers.